There have been known color or monochromatic imaging technologies in which an ink containing a thermally diffusible dye capable of diffusion transfer upon heating, is placed to face the dye receiving layer of an image receiving sheet, after which the thermally diffusible dye is allowed to be imagewise transferred to the dye receiving layer by heat-printing means such as thermal heads or lasers to form an image (employing a so-called thermal dye transfer system). Such a thermal transfer system enables one to achieve image formation using digital data without using processing solutions such as a developer solution. This thermal transfer system is recognized as a method for forming high quality images equal to those of silver salt photography.
There have been proposed various thermal dye transfer methods. Of these, a method of forming various types of full color images has been proposed, in which using a thermal transfer sheet having a sublimation type dye provided on a substrate sheet, the sublimation dye is transferred to a receiving material capable of being colored by the dye, that is, a so-called thermal transfer image receiving sheet having a dye receiving layer which is formed on paper, plastic film or the like. In that case, the thermal head of a printer is used as a heating means and three or four color dots are transferred to the thermal transfer image receiving sheet through heating over an extremely short period, while controlling the heating amount, and the thus multicolor dots can reproduce the full color of a manuscript.
The thus formed image is extremely clear and exhibits superior transparency and is also superior in reproduction or gradation of intermediate colors, whereby image quality equivalent to that of images obtained in conventional off-set printing or gravure printing can be achieved, enabling formation of high quality images equal to full color photographic images.
However, the thus obtained images were proved to have shortcomings such that the image storage stability or fastness was inferior to conventional silver salt photography.
Specifically, the following matters are cited:    (1) image fading or bleeding is caused by light or heat, aerial oxygen, or moisture during storage over a long period of time (light stability and heat stability),    (2) when brought into contact with substances exhibiting relative high dying affinity, such as a photoalbum or clear file, or plastic erasers, or plasticizer-containing materials, dyes are reversely transferred or images bleed out (plasticizer resistance),    (3) when water, juice, wine or coffee is dropped onto formed images and is wiped therefrom, dissolved dyes are also wiped off (water resistance and solvent resistance),    (4) when touched with a finger, the touched portion is discolored due to sebum (sebum resistance),    (5) when rubbed with eraser, image portions are removed (abrasion resistance), and    (6) when converted by using commercially available laminating material, specifically, cold laminate material convertible at a relative low temperature, dyes diffuse into the laminating material, causing bleeding of images (laminate suitability).
Dyes used in conventional silver halide photography are protected with high boiling solvents or ultraviolet absorbents. On the contrary, dyes used in thermal transfer recording material are mainly dispersed in a binder and tend to be directly influenced by an external environment.
There were proposed, as a means for improving the foregoing disadvantages, image forming methods by employing so-called reactive dyes in which a compound contained in the dye layer is allowed to react with a compound in the dye receiving layer through thermal transfer. Herein, when the compound contained in the dye layer and the compound in the dye receiving layer are defined as a dye precursor and a dye fixer, respectively, and JP-A No. 9-327976 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication), U.S. Pat. Nos. 4,880,769 and 5,534,479, for example, proposed that using a deprotonated cationic dye as a dye precursor and an organic polymer or oligomer capable of protonating the cationic dye as a dye fixer, the cationic dye wwes protonated again to achieve image formation. JP-A No. 5-221151 proposed an image forming method in which a reactive group containing a dye with a specific structure as a dye precursor and an active hydrogen compound as a dye fixer are used to perform thermal transfer to form an image.
There was proposed a thermal transfer image receiving sheet including a metal ion containing compound (also called a metal source) capable of forming a chelate upon reaction with a thermally diffusible dye which is chelatable with a metal, as disclosed in JP-A Nos. 10-129126 and 5-4460. Further, a post-chelate sublimation imaging method using a post-chelate type thermally diffusible dye capable of forming a chelate with a metal was also disclosed, for example, in JP-A Nos. 5-301470, 5-177958 and 5-312582, resulting in greatly enhanced image lasting quality, as compared to conventional sublimation images.
Recently, a technique for enhancing print speed (high-speed printing) to shorten the print-out time per sheet has been desired in the foregoing imaging method using thermal dye transfer. In response to such a demand, various studies have been made not only in thermal transfer ink sheet but also in thermal transfer image receiving sheets. There have also been made various attempts for enhancing the print speed in thermal transfer ink sheets and thermal transfer image receiving sheets of post chelate sublimation images exhibiting superior image lasting quality. However, it was proved that enhancing the print speed caused lowering in maximum density or deteriorated light fastness. Therefore, no method which concurrently satisfies enhancement of the print efficiency by shortening the print time and image characteristics (e.g., image density, light fastness) has been discovered under present conditions.
Further, it is contemplated to adopt a method of increasing the dye content of a thermal transfer ink sheet or performing the thermal transfer at relatively high energy to obtain sufficient image densities in the high-speed print. However, increasing the dye content of a thermal transfer ink sheet produced problems that bleed-out of the dye resulted after storage over a long period, resulting in staining of a thermal head and shortening the lifetime of the thermal head. Thermal transfer at relatively high energy often causes fusion between the ink sheet and the image receiving sheet at the time of thermal transfer, resulting in abnormal transfer.